Lesson 22 · The executor and CREATE TABLE

The executor and CREATE TABLE

The parser makes trees; the executor makes them happen. Today you build the executor and its first case - running a CREATE TABLE node to add a real table to the database.

The goal

Execute a parsed CREATE TABLE statement so the named table with its schema exists in the database.

Start here - the target

TO DO

Scenario: Executing a CREATE TABLE statement

Givenan empty database and a parsed CreateTable for "users" with columns (id INTEGER), (name TEXT)

Whenthe statement is executed

Thenthe database has a table "users" with that two-column schema

Andexecuting the same CreateTable again reports a duplicate-table error

Background

The executor is the bridge between the syntax tree and the storage from chapter 1. It is one function that takes a parsed statement and does what it says, dispatching on the statement’s type. The first and simplest case is CREATE TABLE: pull the name and columns off the node and call the CreateTable method the database already has.

Notice how little glue this takes - the parser produced Column values, the database knows how to register a schema, and the executor just connects them. The duplicate-table error you built on lesson 7 surfaces here automatically, which is the payoff of validating at the storage layer. Every other statement type is another case in this same switch; you will fill them in over the next several lessons.

Make it work

func (db *Database) Exec(stmt Stmt) (ResultSet, error) {
  switch s := stmt.(type) {
  case CreateStmt:
    _, err := db.CreateTable(s.Table, Schema{Columns: s.Columns})
    return ResultSet{}, err
  }
}

CheckpointDONE

The executor runs CREATE TABLE, turning a parsed statement into a real table. Commit and stop here.